The invention relates to a method and a solution for growing charge-transfer complex salts, for instance in via holes during the fabrication of switching devices.
The evolution of the market of data storage memories indicates a growing need for ever-larger capacity: from gigabytes (GB's) to hundreds of gigabytes (100 GB's) or even Terabytes. Flash memory technology has so far been able to fulfil scaling requirements, keeping a reasonable cost per bit, but it is expected that this technology will face severe scaling problems beyond the 45 nm technology node due to fundamental physical limitations.
In this context, resistive switching memories—based on a resistor element that can be programmed in a high and low conductive state—constitute replacement candidates, as their physical switching mechanisms may not degrade with scaling. Among potential resistive memory materials, some promising metallic salts of charge-transfer complexes are currently investigated such as AgTCNQ and CuTCNQ, TCNQ standing for 7,7,8,8-tetracyano-p-quinodimethane. For example, the organometallic material CuTCNQ shows nanosecond electrical resistive switching. These compounds are prepared by a spontaneous chemical reaction of a metal M with a strong electron-acceptor A, leading to the semiconducting charge-transfer salt M+A−:M+A→M+A− (FIG. 1). Cu+ TCNQ− can be prepared by dipping a copper substrate in a solution of TCNQ in acetonitrile (CH3CN) at room temperature as described by Potember et al in Appl. Phys. Lett. 34, 405 (1979).
The global reaction (eq. 1) called “spontaneous electrolysis” consists in the corrosion of the copper substrate by dissolved TCNQ, resulting in the formation of Cu+TCNQ− salt which has a relatively low solubility and deposits on top of the copper as a multicrystalline layer.Cu+TCNQCH3CN→Cu+TCNQ−  (eq. 1)                Nitrile solvents, as for example acetonitrile, are required for this reaction because they stabilize the usually unstable Cu+ cation by coordination, which will be symbolized for acetonitrile solvent by Cu+CH3CN.        
The global equation (eq. 1) consists in two steps: a simple electron transfer (oxidation-reduction) between the copper substrate and the dissolved TCNQ (symbolized by TCNQCH3CN, eq. 2) generating CU+CH3CN cations and TCNQ−CH3CN anions, followed by (partial) co-precipitation of these two ions at the copper/solution interface as Cu+TCNQ− crystals (eq. 3).Cu+TCNQCH3CNCU+CH3CN+TCNQ−CH3CN  (eq. 2)Cu+CH3CN+TCNQ−CH3CNCu+TCNQ−  (eq. 3)
Both reactions in eq. 2 and eq. 3 are equilibrated (represented by the symbol ‘’), but due to the large difference in standard electrode potentials (E0′) of the electrochemical couples the electron transfer reaction (eq. 2) is completely shifted towards the right side of the equilibrium (formation of the ions CU+CH3CN and TCNQ−CH3CN).
The second step in the formation of Cu+TCNQ− crystals at the copper/solution interface (eq. 3) is a precipitating reaction depending upon the local concentrations of [CU+CH3CN] and [TCNQ−CH3CN] (both in mol/L). Crystals of Cu+TCNQ− are deposited at the copper surface when the product of both local concentrations is higher than the constant Ksp, called the “solubility product” (eq. 4):[CU+CH3CN].[TCNQ−CH3CN]>Ksp  (eq. 4)Harris et al. reported in J. Electrochem. Soc. (2005) 152, C577, values for Cu+TCNQ− solubility at room temperature in pure acetonitrile (0.14±0.05 millimol/L), and in acetonitrile in presence of 0.1 mol/L n-butylammonium hexafluorophosphate salt (0.7±0.3 millimol/L). Since the concentrations [CU+CH3CN] and [TCNQ−CH3CN] are equal in a saturated Cu+TCNQ− solution in pure acetonitrile, the computed solubility products at room temperature are respectively 2·10−8 mol2/L2 and 4.9·10−7 mol2/L2 in absence and in presence of the 0.1 mol/L n-butylammonium hexafluorophosphate salt.
Although this “spontaneous electrolysis” reaction could in principle be performed on copper metal at the bottom of via-size contact holes, corresponding samples show extensive corrosion of the copper so that often even the copper interconnection near the via hole was significantly corroded and very often even interrupted (FIG. 3).
Without being bound by theory, the difference in behavior observed for the reaction between a copper substrate and a TCNQ solution in acetonitrile upon downscaling to sizes typical of via holes can be explained by a change in diffusion regime of the TCNQCH3CN when the size of the metal surface becomes small. In fact, in electrochemical measurements it appeared that the mass transfer changes from planar diffusion to non-planar diffusion when the size of the electrode is decreased (FIG. 5). During this change to non-planar diffusion the current density increases, which can be attributed to an increase of flux at the electrode-solution interface. This effect has also been observed for the diffusion limited current measured in electrochemical experiments at recessed microdisc electrodes, which are similar to the via structure described herein. Since the formation of CU+CH3CN and TCNQ−CH3CN proceeds also by an electron transfer reaction (this time at a copper layer and with the difference that the electrons are not originating from an external circuit but from the copper itself, a similar increase of flux upon downscaling of the electrode may be observed as in the case of an electrochemical process. The corresponding increase in flux is not only valid for the species diffusing towards the electrode (TCNQ), but also for species generated at the electrode (TCNQ−CH3CN and CU+CH3CN). Due to the enhanced flux both species are diffusing fast away from the copper present on the bottom of the via-hole such that the kinetics of the precipitation reaction (eq. 3) becomes too slow for the deposition of crystalline Cu+TCNQ−, resulting in extensive corrosion of the Cu metal on the bottom of the via. Different methods for achieving this spontaneous chemical reaction have been described, but these methods exhibit problems for controlling the growth of charge-transfer complex salts M+A−, for instance inside small volumes such as e.g. via holes of a CMOS back end-of-line wafer with metal M at the bottom of the vias (FIG. 2).
R. Müller et al describes in communication EP-2 of the 1st International Conference on Memory Technology and Design (ICMTD), Giens (F), May 21-24, 2005, a method wherein polycrystalline layers of CuTCNQ are formed on top of a patterned metal by placing the metal M in a solution of the acceptor A in an organic solvent (e.g acetonitrile or n-butyronitrile) at room temperature or at an elevated temperature (i.e. above room temperature). This method is not suitable for the growth of the semiconducting material M+A− inside vias since the reaction between a sub-micrometer sized metallic element M with an acceptor A in liquid organic solvents is generally difficult to control, leading to uncontrolled growth of the M+A− salt outside the via as well as to corrosion of the metallic connections beneath the via (FIG. 3).
An alternative preparation method consists in co-evaporation of the metal M and the acceptor A (mostly in stoechiometrical amounts), giving amorphous layers of the semiconducting memory material M+A− on the whole exposed area. With this method, the stoechiometry is difficult to control when the metal M and the acceptor A are co-evaporated, and furthermore deposition of the charge-transfer complex salt M+A− occurs also outside the vias.
Also, M+A− wires can be grown in 250 nm diameter vias of a Cu CMOS back end-of-line wafer via the reaction of the solid metal M (deposited or patterned on a substrate) with the acceptor A in the gaseous state. The diameter and length of sub-micrometer sized semiconductor wires, resulting of the reaction of the solid metal M with vapor of the acceptor A, are difficult to control so that some via holes are only partly filled by the memory material M+A− and the wires are growing far outside the via. This can be an issue for a subsequent planarization step undertaken before deposition of top contacts and for reproducibility of the electrical switching characteristics (switching voltages and currents).
Vapor deposition of the acceptor A on the metal M followed by treatment with vapor of an organic solvent has been reported to lead to semiconducting layers. Preparation of the memory material M+A− by sublimation of the acceptor A on metal M on the bottom of the via hole, followed by inducing the reaction between both reagents by treatment with an organic solvent vapor, is also problematic since first all exceeding acceptor A outside the vias has to be removed before treatment with solvent vapor in order to avoid uncontrolled growth of the M+A− salt outside the via and second, corrosion of the metallic connections beneath the via occurs.
Finally embedding of the charge transfer materials M+A− inside a continuous solid phase (matrix) has been reported. Examples are switching devices from an organic charge-transfer salt prepared from a TCNQ polymer and fusible mixtures for melt coatings. The use of polymer based materials or fusible mixtures are challenging on two points: filling of the vias and consecutive polishing in order to remove material between the vias. Furthermore this kind of material should, due to the presence of the matrix, exhibit lower switching currents, and also lower reading currents compared to monocrystalline memory materials.
There is therefore a need in the art for method and solutions to grow charge complex salts M+A− in small size holes, e.g. in submicrometer diameter via holes on a CMOS BEOL wafer or a similar substrate.